GPS receiver with efficient signal acquisition

ABSTRACT

A direct sequence spread spectrum receiver samples an incoming signal and translates the signal to an IF signal. The IF signal is sampled and stored in memory. In one embodiment, the memory consists of two memory banks which alternately receive sample segments. During a write period to one of the memory banks, the other memory bank supplies its output to a processor. This continues in a ping-pong manner. In another embodiment, a single memory bank is filled and read as necessary, the receiver ignoring incoming signal until the processor has completed processing the sample available at the output of the memory. Such a receiver is useful in global positioning satellite (GPS) signal processing where the incoming signal contains several satellite transmissions encoded with CDMA encoding.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional ApplicationSerial No. 60/021,628, filed Jul. 12, 1996.

This application discloses subject matter related to that of abandonedapplication Ser. No. 08/456,229 filed on May 31, 1995 now U.S. Pat. No.5,752,218, issued May 12, 1998, by Daniel D. Harrison, Anantha K.Pradeep, Glen W. Brooksby and Stephen M. Hladik for "A Reduced-PowerGPS-Based System for Tracking Multiple Objects from a Central Location"and assigned to the assignee of this application. The disclosure of U.S.Pat. No. 5,752,218 is incorporated herein by reference.

This application is related to co-pending applications Ser. Nos.08/883,162; 08/883,161; 08/883,163; 08/883,421, 08/883,420; 08/883,419;08/883,160 filed concurrently herewith and assigned to the assignee ofthis application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to spread spectrum communicationssystems and, more particularly, to a low-power signal processingarchitecture and method for spread spectrum receivers.

2. Background Description

Spread spectrum communication is advantageous in communicationapplications requiring high reliability in a noisy environment. Oftenthe dominant noise is man-made interference, either intentional oraccidental. In a specific application the communication environment mayinclude many potential reflectors, giving rise to severe multi-pathinterference. Such multi-path interference typically insinuates deepnulls in the form of frequency selective fading. Spread spectrumcommunications is an excellent countermeasure to these difficulties.

There are several types of spread spectrum systems including directsequence spread spectrum systems, frequency hopping systems, timehopping systems, pulse frequency modulated (or chirp) systems, andvarious hybrids. Of these, the direct sequence spread spectrum (DSSS)systems and frequency hopping systems are perhaps the more widelyimplemented. The following discussion is focused on binary DSSS systems.

In binary DSSS communication, a wide band carrier signal is modulated bya narrow band message signal. The wide-band carrier is typicallygenerated by bi-phase modulating a single frequency carrier using abinary pseudo-random noise (P/N) code sequence. The P/N code is oftengenerated using one or more high speed shift registers, each havingmodulo-two feedback according to a primitive polynomial. The generatedhigh-speed P/N code is then applied to a balanced modulator (multiplier)whose other input signal is the narrow band carrier. The output signalof the balanced modulator is a wide-band signal often referred to as a"wide-band carrier". To communicate data, the wide-band carrier isbi-phase modulated by a binary message data stream. The message datarate is usually much lower than the P/N-code symbol or "chip" rate, andthe data and code-chip edges are usually synchronized. The ability ofthe DSSS technique to suppress interference is directly proportional tothe ratio of the code-chip rate to the data rate. In many applications,there are thousands of code chips per message bit.

A DSSS signal can be received by first shifting the signal down tobaseband by multiplying it with a locally generated replica of theoriginal narrow-band carrier (e.g., a properly tuned local oscillator).If the frequency (and phase) of the carrier replica is the same as thatof the received original narrow-band carrier, then the multiplier outputsignal will be a bipolar "wide-band data" stream that is the product ofthe bipolar P/N code and message-data sequences. The P/N code is thenremoved by multiplying the wide-band data stream with a locallygenerated replica of the P/N code that is time aligned with the receivedP/N code. This is the data de-spreading process and yields the originalmessage data stream at the multiplier output.

In the data de-spreading process, the wide-band data power spectrum isrefocused into the original narrower data bandwidth, raising the datapower level well above the background noise in that bandwidth. Theamount that the power level is raised is the so called processing gainand is directly proportional to the ratio of the code rate to the datarate. Furthermore, any received narrow-band interference is spread bythe code-replica modulation, and this greatly reduces the interferencepower level in the data band.

An often difficult task associated with DSSS signal reception is that ofgenerating the carrier replica with both proper carrier frequency andphase and generating the P/N code replica at the proper rate and withproper time alignment (offset). In many DSSS communication systems, thenecessary carrier frequency, carrier phase, and P/N code offset are notknown a priori at the receiver and these parameters must be determinedby trying different values until a large signal is observed at thedata-filter output. This is known as the search or acquisition process,and a DSSS signal is said to be acquired when the proper frequency,phase, and code offset have been determined.

In many DSSS applications, the DSSS signal levels are well below thebackground noise and/or interference levels and are not detectable untilproperly de-spread and low-pass filtered. When the receivedsignal-to-noise ratio (SNR) is very low, the filter must be very narrowto achieve the processing gain needed for signal detection andacquisition. Because a narrow filter requires a long integration period,the result of multiplying many received P/N code samples by thecorresponding replica P/N code samples must be accumulated before thedetection decision can be made. This multiplication and accumulation isa cross correlation between the received and replica P/N code sequences,and the sequences may have to be long for low SNR signals.

Use of the DSSS method enables multiple users to simultaneously sharethe same wide-band channel using the code-division multiple access(CDMA) technique. With this technique, each transmitter utilizes adifferent P/N code such that the cross correlation between differentcodes is substantially zero. A receiver selects and detects a particulartransmitted signal by choosing the appropriate P/N code and performingthe acquisition search. In some cases, it is unknown which transmittermay be transmitting and the acquisition search must include examinationof different P/N codes from a known list. When many different codes,code offsets and carrier frequencies must be examined and the SNR islow, the acquisition task can be both time and energy consuming. Animportant aspect of the present invention is the reduction of the timeand energy consumed in the DSSS signal acquisition process.

A description of direct sequence and other types of spread spectrumcommunications systems may be found, for example, in Spread SpectrumSystems, 3^(rd) Ed., by Robert C. Dixon, John Wiley & Sons (1994), andSpread Spectrum Communications, Vol. II, by M. K. Simon et al., ComputerScience Press (1985). A description of CDMA techniques may be found, forexample, in CDMA Principles of Spread Spectrum Communication, by AndrewJ. Viterbi, Addison-Wesley (1995).

The popular and ubiquitous Global-Positioning System signals are animportant application of DSSS communications. In recent years, NavstarGlobal-Positioning System (GPS) satellites have been launched intomedium-altitude earth orbits in six orbital planes, each tipped 55° withrespect to the equator. The complete GPS satellite constellationcomprises twenty-one satellites and several spares. Signals transmittedfrom these satellites allow a receiver near the ground to accuratelydetermine time and its own position. Each satellite transmits data thatprovides precise knowledge of the satellite position and allowsmeasurement of the distance from that satellite to the antenna of theuser's receiver. With this information from at least four GPSsatellites, the user can compute its own position, velocity and timeparameters through known triangulation techniques (i.e., the navigationsolution). Typically, seven, but a minimum of four, satellites areobservable by a user anywhere on or near the earth's surface if theuser's receiver has an unobstructed view of the sky, down to very nearthe horizon. Each satellite transmits signals on two frequencies knownas L1 (1575.42 MHz) and L2 (1227.6 MHz), and all satellites share thesefrequencies using the CDMA DSSS techniques described earlier.

More particularly, each satellite transmits a single high-resolutionDSSS signal on frequency L2 and the same signal plus anotherlower-resolution DSSS signal on frequency L1. The low-resolution DSSSsignal comprises a P/N code with a 1.023 MHZ chipping rate and a 1.0 msrepetition period, and a message data sequence (the NAV data) with arate of 50 bits per second. The high-resolution DSSS signal uses a P/Ncode with a 10.23 MHz chipping rate and a repetition period longer thana week. The same NAV data stream is used in all DSSS signals from agiven satellite. The NAV message from a given satellite contains the GPSsignal transmission time, ephemeris (position) data for that satellite,almanac data (a reduced accuracy ephemeris) for all of the satellites inthe constellation, and a hand-over word used in connection with thetransition from low-resolution to high-resolution code tracking. The lowand high-resolution codes are known as the course/acquisition (C/A) andprecise (P) codes, respectively.

After acquisition, the offset of each code, together with thesignal-transmission time from the NAV data, enables a receiver todetermine the range between the corresponding satellite and the user. Byincluding both the P code and the repeating C/A code in the transmittedsignal, a more-rapid hierarchical acquisition of the P code is madepossible and a two tiered level of global navigation service can beprovided. The P code can provide positions that are accurate toapproximately 3 meters, while the C/A code yields accuracies on theorder of 30 meters. Typically, the low-resolution service isunrestricted while the high-resolution service is restricted to themilitary by encrypting or otherwise controlling knowledge of thehigh-resolution P/N code.

In a typical military receiver, the C/A code is acquired first. Then thehand-over word is read from the NAV data stream. The hand-over wordspecifies the approximate offset of the P code relative to GPS time (astransmitted in the time stamp), and its use will dramatically reduce thenumber of different code offsets that must be searched during the P codeacquisition. Acquisition of the C/A code is substantially easier thandirect acquisition of the P code because the C/A code repeats every 1.0ms and there are, therefore, only 1023 different code offsets to search(twice this if the search is performed in the usual half-chip steps).

Received GPS signals are usually shifted in frequency from the nominalL1 and L2 carrier frequencies because the GPS satellites move in orbitat several kilometers per second, yielding a substantial Doppler shift.The satellite trajectories are usually known a priori and the Dopplershifted carrier frequencies are therefore predictable if the GPSreceiver location is known. Unfortunately, the receiver location is notknown a priori, and there is often substantial local oscillator errorwith inexpensive receivers. The resulting uncertainty in receivedcarrier frequency (i.e., in needed replica carrier frequency) can belarge (e.g., ±7.5 kHz), and this frequency range may have to be searchedduring the GPS signal-acquisition process. The frequency or Dopplersearch is usually done by repeating the cross correlation of thereceived sample and local replica P/N sequences for different localoscillator (carrier replica) frequencies. The spacing between frequencysteps is made small enough to avoid missing the signal when longcross-correlation integration times (narrow filter bandwidths) are used.Long integration times improve detection of low SNR signals. Withtypical civilian GPS applications, 1.0 millisecond cross-correlationintegrations are used (a single C/A code cycle), yielding an equivalentDoppler filter bandwidth of approximately 500 Hz. A ±7.5 kHz frequencyrange can be searched with thirty 500 Hz steps. The GPS acquisition thenentails a search over satellite code, code offset, and Dopplerfrequency.

A master control station (MCS) and a number of monitor stations comprisethe control portion of the GPS system. The monitor stations passivelytrack all GPS satellites in view, collecting ranging data and satelliteclock data from each satellite. This information is passed to the MCSwhere the satellites' future ephemeris and clock drift are predicted.Updated ephemeris and clock data are uploaded to each satellite forre-transmission in each satellite's NAV message.

In operation, a typical GPS receiver performs the following for each ofat least four satellite signals:

1) acquires the DSSS signal,

2) synchronizes with the NAV data stream and reads the satellitetime-stamp, clock-correction, ionospheric-delay and ephemeris data,

3) calculates the satellite position from the ephemeris data,

4) reads its own receiver clock to determine the receiver timeassociated with the reception of the time-stamp epoch, and

5) estimates the signal travel time by subtracting the time-stamp valuefrom the associated receiver time.

This time difference is multiplied by the speed of light to obtain anestimated range to the satellite. If the GPS receiver had a clock thatwas perfectly synchronized with the clocks of the satellites (or theerror was known), only three such range estimates would be required toprecisely locate the receiver. There is, however, a clock-bias (slowlychanging error) due the fact that GPS receivers typically useinexpensive crystal clocks, whereas the satellites are equipped withatomic clocks. This clock bias is learned and its effect eliminated bymeasuring the range (travel time) from four GPS satellites and usingthese measurements in a system of four equations with four unknowns(receiver x, y, and z, and time). For general information on GPS, thereader is referred to the book by Tom Logsdon entitled The NavstarGlobal Positioning System, by Van Nostrand Reinhold (1992).

A preferred application of the present invention is the locating andtracking of assets such as rail cars, shipping or cargo containers,trucks, truck trailers, and the like, using the GPS. In thisapplication, the GPS receivers are usually battery powered since anindependent source of power is generally not available. It isadvantageous to increase the operating life of the batteries by reducingenergy consumed by the GPS receiver.

In a typical spread spectrum receiver, the receiver front end (i.e., RFand IF electronics) consumes a large amount of power while it is turnedon. This results in high energy consumption if the signal acquisitionand synchronization take a long time. Most prior-art GPS receivers donot have signal storage (memory) and must process the received signalsin real time. Furthermore, they use either a sequential search or searcha small number of satellite/code-offset/Doppler (SCD) binssimultaneously to achieve signal acquisition. Such receivers mustcontinually receive and process each satellite signal until its SCD binis identified and the necessary NAV data is decoded. With a sequentialsearch the energy consumption is high because substantial time iselapsed before the SCD bin associated with each GPS signal isidentified. Alternatively, multiple SCD bins can be searched in parallelto reduce the elapsed time, but the energy consumption is still highbecause the existing processing methods are not very low power methods.Furthermore, the degree of parallelism is very limited with existingprocessing methods due to the large amount of circuitry involved.

In one system of the invention, a central facility or station must trackmultiple assets (e.g., railcars). Each tracked object carries a GPSreceiver that processes data from several of the visible GPS satellites;however, an accurate position determination is not made at the receiver.Instead, only partial processing is done at the receiver andintermediate results are transmitted from the asset to the centralstation. These intermediate results do not require decoding ofnavigational or other data from the GPS signals. This system thus allowsthe GPS receiver and signal processor to be powered only long enough toacquire the satellite signals (determine the SCD bins). With thissystem, the dominant energy consumer is the acquisition process, and theGPS receiver energy used at each tracked asset will be dramaticallyreduced if the signal acquisition time and energy are dramaticallyreduced.

U.S. Pat. No. 5,420,593 to Niles uses a memory to store an interval ofthe received signal containing multiple GPS satellite signals. Thereceived signal is sampled and written into the memory at one rate andthen read from the memory at another, faster rate. Upon reading, thesignal is digitally processed to acquire and synchronize with thereceived GPS satellite signals. This allows a shorter elapsed time forthe acquisition of the GPS signals. However, the receiver is not turnedoff immediately after signal storage, and low-power signal acquisitionis not used. Furthermore, substantially reduced energy consumption isnot achieved.

U.S. Pat. No. 5,225,842 to Brown describes a GPS based centralized assettracking system that reduces the cost of the GPS receivers on eachtracked asset by avoiding calculation of the navigation solution at theasset. Each asset carries a GPS receiver that processes the signal fromseveral of the visible GPS satellites and relays the processed result tothe central station where accurate asset navigation solutions arecalculated. This system does not substantially reduce the energyconsumed by the GPS receiver at the asset and does not substantiallyextend asset battery life or reduce time between service to replacebatteries. Furthermore, low-power parallel correlation is not used.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a direct sequencespread spectrum (DSSS) signal processing architecture which permits thereceiver to be turned off during most of the acquisition phase ofreception, thereby permitting the on time of the associated receiverfront end to be significantly reduced.

Another object of the invention is to provide a signal processingarchitecture which permits low power consumption during the acquisitionphase of DSSS signal reception.

A further object of the invention is to provide a low-power parallelcorrelation method that is readily manufactured with availableintegrated circuit processes and utilizes low energy to acquire DSSSsignals.

Another object of the invention is to provide a GPS signal processingarchitecture which permits P/N code and Doppler searches (associatedwith the acquisition process) to be performed with a standard sequentialprocessor (i.e., relatively slowly and with a small processing resource)without requiring the receiver front end to be turned on during thesearch process.

Yet another object of the invention is to provide a GPS signalprocessing architecture that requires little energy to track an assetusing the GPS.

Still another object of the invention is to provide a signal processingarchitecture that can dynamically exchange coherent and non-coherentintegration times as is needed for a particular received signal-to-noiseratio (SNR).

A still further object of the invention is to employ a low receiveroutput sampling rate in obtaining accurate sub-chip DSSS signalacquisition timing.

Yet another objective of the invention is to provide a GPS signalprocessing architecture exhibiting rapid acquisition of GPS satellitesignals.

Still another objective of the invention is to provide a GPS signalacquisition method that allows rapid and low-energy signal acquisitioneven when the GPS receiver uses an inexpensive local oscillator that maybe inaccurate.

According to one aspect of the invention, a low-power high-speedparallel correlator is used during DSSS signal-acquisition to reduce theacquisition energy consumption. Receiver energy consumption is furtherreduced because the time that the receiver must be in the on conditionis reduced due to the speed of the parallel correlator.

According to another aspect of the invention, a time interval ofreceiver output data is sampled and stored in a memory and then thereceiver is turned off. The time interval is long enough to allowacquisition of any of the received CDMA DSSS signals contained in thestored receiver output data. The stored receiver output data isre-played from memory as many times as may be required to acquire eachdesired CDMA signal. To keep processing energy consumption low, alow-power parallel (partial analog) correlator may be used in theacquisition process. This approach uses much less energy than existingdigital cross-correlators.

The centralized GPS tracking system according to the invention allowsthe tracked assets to consume very low power and use a receiver with aninexpensive local oscillator. The use of a parallel correlator allowsthe local oscillator to be inaccurate because many frequencies arereadily searched during the acquisition process using low power.Furthermore, use of the parallel correlator allows reception anddemodulation of the GPS NAV data to be avoided, further reducing theaverage on time of the receiver. The almanac is no longer needed tosupport a reduced satellite search time because the parallel correlatorcan quickly search over all satellite codes. Since the navigationsolution is not needed at the tracking units, only limited processingconsuming very low power is needed, the navigation solution beinggenerated at a central station.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are set forth in theappended claims. The invention, however, together with further objectsand advantages thereof, may best be understood by reference to thefollowing description taken in conjunction with the accompanyingdrawing(s) in which:

FIG. 1 is a block diagram of a remote tracking system in accordance withthe present invention;

FIG. 2 is a block diagram of a tracking unit on an object to be trackedin accordance with the invention;

FIG. 3 is a block diagram of a conventional sequential acquisitionarchitecture;

FIG. 4 is a block diagram of the sequential signal processingarchitecture according to one embodiment of the invention;

FIG. 5 is a block diagram of the parallel signal processing architectureaccording to another embodiment of the invention;

FIG. 6 is an illustration of the parallel correlation process betweenthe stored-data and sliding replica waveforms.

FIG. 7 is a block diagram showing a structure for generating the replicasequence;

FIG. 8 is a block diagram showing a short segment of an embodiment ofthe data memory, replica register, and parallel correlator that uses thereplica generation method of FIG. 7;

FIG. 9 is a block diagram showing one embodiment of one stage of thetwo-sequence parallel correlator of FIG. 8;

FIG. 10 is a block diagram showing a method for generating separate codeand Doppler replica sequences;

FIG. 11 is a block diagram showing an embodiment of the data memory,code-replica register, Doppler-replica register, and parallel correlatorthat uses the replica generation method of FIG. 10;

FIG. 12 is a block diagram showing one embodiment of one stage of thethree-sequence parallel correlator of FIG. 11;

FIG. 13 is a schematic diagram illustrating the principles of thedifferential digital-to-analog converter and summation according to oneaspect of the invention;

FIG. 14 is a graph showing the noiseless and noisy auto-correlationwaveform for a short section of the 1.0 ms C/A-code P/N sequence nearthe correlation peak;

FIG. 15 is a block diagram showing a low-power parallel-correlator basedacquisition system with full I/Q processing in which the RF/IF sectiongenerates two quadrature outputs;

FIG. 16 is a block diagram of one embodiment of a squaring A/Dconverter;

FIG. 17 is a block diagram of a combined data-memory,replica-generation, parallel-correlator organization that reduces thenumber of code register shifts per code/Doppler combination.

FIG. 18 is a schematic diagram of a three-sequence multiplierdigital-to-analog converter cell;

FIG. 19 is a schematic diagram of a Ping-Pong data-memory organization;

FIG. 20 is a schematic diagram of a parallel correlator withdata-Doppler pre-multiplication;

FIG. 21 is a block diagram of a full I-Q processor with data-Dopplerpre-multiplication; and

FIG. 22 is a block diagram of a full I-Q processor with data-Dopplercomplex pre-multiplication.

FIG. 23 is a block diagram of the low-energy register writing methodusing a scanner shift register.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 illustrates a plurality of GPS satellites 12, an object (asset)being tracked, such as a rail car carrying a tracking unit 14, and acentral station 16. As described earlier, each satellite 12 transmits asignal that a GPS receiver in tracking unit 14 uses to measure thepropagation delay (and delay rate, if asset velocity is desired) fromthat satellite to the receiver antenna. The satellite signals alsoinclude the periodically repeating NAV data that is needed to determinea navigation solution from the measured time delays. Because of the lowrate (50 bits per second) of the NAV data in the GPS signal, a receivermust be active for a substantial period of time (from one to severalminutes) if the NAV data is to be collected. Furthermore, a particularsatellite's NAV data changes over time and the GPS MCS monitors thesechanges and provides nearly hourly updates to the NAV data. To insureaccurate navigation solutions, any GPS based navigation system must useNAV data that is no more than about four hours old. If the assetpositions are to be monitored more frequently than every four hours,then new NAV data will have to be collected at least every four hours.NAV data maintenance then requires an approximate average of fifteenseconds of receiver operation per hour, and this entails a significantenergy requirement if done at each tracked asset.

According to one aspect of the present invention, the navigationsolution is calculated at the central station instead of at the asset.None of the NAV data is needed at the tracked asset. Only data relatingto the GPS signal propagation delay between each satellite and the assetneed be measured at the asset, and this data is then sent to the centralstation. The NAV data can be determined at central station 16 byutilizing a standard GPS receiver there, or by communicating with anappropriately located standard GPS receiver. If desired, the NAV data,or the navigation solutions, can be communicated to the tracked assetsvia a higher rate communication link that requires less energy toreceive at the asset. With no NAV data decoding needed at the asset, GPSsignal acquisition becomes the main GPS processing task at the asset,and the feasibility of the centralized tracking system is greatlyenhanced by the low-energy acquisition methods of the invention.

As shown in FIG. 2, a rail car tracking unit 14 is comprised of areceiver 2 responsive to the signals received at an antenna 5 from theGPS satellites, a processor 3, and a transmitter 4. The received signalsare processed in processor 3 to ascertain and utilize propagation timedifferences among the signals received from the GPS satellites. Byutilizing time differences, the need for knowledge of the GPS signaltime-stamps at the asset is alleviated, and data stream decoding istherefore unnecessary at the asset. With no need for GPS data streamdecoding, the receiver process reduces to acquisition of the GPS signalsand calculating the relevant time differences (and Doppler frequencydifferences, if asset velocities are to be determined) from theacquisition results. The calculated time differences, and dataidentifying the satellites associated with the differences, aretransmitted by transmitter 4 from an antenna 6 to the central station. Adescription of the centralized tracking system in which GPS signalacquisition is a major part of the assets' GPS energy budget is found inco-pending application Ser. No. 08/456,229 of Harrison, Pradeep,Brooksby and Hladik, referenced above.

FIG. 3 shows a conventional serial acquisition architecture that uses aserial correlator. In a conventional GPS receiver, signal acquisition isfollowed by carrier and P/N code synchronization and NAV datademodulation, but modules for these processes are not shown in FIG. 3.The signal acquisition architecture comprises an RF/IF (radiofrequency/intermediate frequency) tuner section 21 including an antenna211, an RF amplifier 212, a mixer 213 and local oscillator 214, and alow pass filter 215 which supplies a received and down-converted signalto analog-to-digital (A/D) converter 22. A/D converter 22 typicallysamples and converts at an integer multiple of the replica C/A code chiprate and supplies a digital sequence to a serial digital correlator 23.Correlator 23 serially computes the inner product of a digitizedreceiver-output sub-sequence from A/D converter 22 and a C/A replicacode sub-sequence originated from a code/Doppler (or replica) generator24. The inner product is performed serially by first multiplying thefirst terms of the two sub-sequences in a multiplier 231 and storing theresult in a coherent accumulator 232, then multiplying the second termsof the two sub-sequences and adding their product to coherentaccumulator 232, etc. The inner product is performed in real time, assub-sequence terms become available from A/D converter 22. As isconventional, the sub-sequences usually span a single period of therepeating C/A code. After an inner product has been computed, thecoherent accumulator contains a sample of the cross correlation of thereceived sub-sequence with one C/A code cycle of the replica signal, forthe particular C/A code, code-offset and Doppler frequency generated bythe replica generator. The inner product operation is repeated withseveral following sub-sequences from A/D converter 22 while using thesame replica C/A code sub-sequence. The subsequent inner product resultsare then squared by a squarer 29 and summed in a non-coherentaccumulator 30, yielding a non-coherent integration of the coherentprocessing results. The output signal of non-coherent accumulator 30 isexamined by a threshold detector 31 and a "signal acquired" command isgenerated if the signal level in non-coherent accumulator 30 issufficiently high. When a signal is acquired, a control 27 monitors theassociated C/A code index (satellite index), code offset, and Dopplerfrequency, and commands code/Doppler generator 24 to either change to adifferent C/A code (for a different GPS satellite) and begin anothersearch, or to stop if sufficient satellite signals have been acquired.If a signal is not acquired after the several sub-sequences from A/Dconverter 22 have been processed, then control 27 commands code/Dopplergenerator 24 to change to a different C/A code, code offset, or Dopplerfrequency. As each satellite signal is acquired, control 27 supplies theassociated code index, code offset and Doppler frequency to GPS signalsynchronization and NAV data processing units (not shown).

Provision for searching all feasible C/A codes, code offsets and Dopplerfrequencies must be provided. Control 27 selects the desired C/A codeand code offset through commands to a satellite-code generator 243 andoffset generator 244. Offset generator 244 provides for a time offset ofthe generated code replica from satellite-code generator 243, relativeto the bit stream from A/D converter 22. A Doppler I/Q generator 242generates a digital representation of the sinusoid representing thecombination of Doppler shift and local-oscillator frequency errorassumed by control 27. The replica signal is generated by a multiplier241 as the product of this sinusoid and the C/A code replica. To insuredetection of the GPS signal, the RF/IF section 21 must generate both anin-phase (I) and a quadrature (Q) output signal (not shown in FIG. 3),and the two signals must be processed for signal acquisition.Furthermore, each of the I and Q RF/IF output signals must be processedwith both the I and Q Doppler sinusoidal components, as is known bythose skilled in the GPS art.

The conventional approach shown in FIG. 3 requires the receiver outputdata from RF/IF section 21 to be processed as soon as it is available,and processing is constrained by the code rate in the received signal.The architecture for an improved sequential signal acquisition processoraccording to one aspect of the invention is shown in FIG. 4. Thearchitecture is similar to that of FIG. 3 except that a signal-storagememory 33 is added and power to RF/IF section 21 is now controlled bycontrol 35. The GPS signal supplied by RF/IF section 21 is converted todigital format by A/D converter 22, but now the A/D sampling rate may beset at a low non-integer multiple of the C/A code rate. Memory 33 storesa length of input signal sufficient for signal acquisition and allowsthe RF/IF section to be turned off after storage. The acquisitionprocess then proceeds by reading and possibly re-reading the data storedin memory. Energy consumption is thereby significantly reduced becausethe RF/IF section 21 consumes significant power. Furthermore,acquisition processing is no longer constrained by the code rate of thereceived signal, as noted in prior art. The non-integer input A/Dsampling rate allows the acquisition processor to determine accurate GPSsignal propagation time differences (needed for determining the locationsolution) while using a much lower sampling rate. Also, when the inputSNR is not too low, or when accuracy requirements are not too high, thelow non-integer sampling rate allows the time differences to bedetermined with sufficient accuracy that the conventional carrier andP/N code synchronization processes normally needed (e.g., the Costasphase-locked loop for carrier tracking, and the early-late delay-lockedloop for code tracking) can be avoided.

An advantage of storing the received signal segment in memory andre-reading the memory as needed to process different SCD bins is thatthe correlation process can take place over a period of time withoutloss of signal-acquisition accuracy due to local oscillator instabilityor inaccuracy. Furthermore, if the stored signal is also used to derivethe time delays needed for the navigation solution, there is no need tomaintain accurate timing between the acquisition and tracking phases ofGPS reception. For applications where the navigation solution is notneeded immediately after measurement (e.g., asset tracking), a very lowpower GPS receiver can be constructed by using a very low-powerintegration process for the sequential correlator (and other circuits)in combination with a low-power data storage memory. Very low powerlarge-scale integration processes are being developed in the industry(e.g., 1.5 v, 0.35 micron complementary metal-oxide-semiconductor orCMOS process).

With the GPS system, as in most DSSS systems, the SNR is very low beforesignal processing, and a substantial period of the received signal mustbe processed to yield the high SNR correlation peak needed for reliablesignal detection by threshold detector 31. For normal civilian GPSapplications, approximately 20 ms of signal from the RF/IF section 21must be stored and processed. To keep the memory size small, the signalfrom RF/IF section 21 is sampled at a low rate and is quantized to onlya few levels. For civilian applications, conventional GPS receiverstypically achieve a GPS fix (location) accuracy of 30 meters. GPS fixeswith this accuracy can be calculated from signal code offsets that aremeasured with an error of less than one tenth of a C/A code chip. Thesignal code offset is measured by noting the replica-signal code offsetassociated with the cross-correlation peak. In one embodiment of theinvention, a sequence of results from non-coherent accumulator 30 aregenerated in order of increasing replica code-offset while holding thecode index and Doppler frequency constant. If a large correlation resultis observed, an interpolation algorithm is applied to the sequence ofresults and the code-offset associated with the correlation peak isestimated. A code-offset accuracy of one-tenth of a code chip isachieved while sampling the signal at approximately twice the C/A-codechip rate. Some conventional GPS receivers don't achieve the desiredone-tenth chip accuracy during signal acquisition; instead, some do soduring code synchronization by sampling the signal at twice the C/A-codechip rate and tuning the sampling phase as part of the early-latedelay-locked loop. Alternatively, other conventional GPS receiversachieve one-tenth chip timing accuracy by sampling the signal at tentimes the C/A-code chip rate and noting the code offset, in one-tenthchip increments, that yields the largest correlation peak. A reducedsampling rate requires less memory and lower processing rates.

The SNR at the input to A/D converter 22 is well below zero, so littlesignal degradation is suffered when the A/D converter uses just threeappropriately chosen representation levels. Each sample can beconveniently encoded in sign-magnitude format using only two data bits.To insure signal detection, both I and Q RF/IF output signals must begenerated, stored, and processed (only one RF/IF channel is shown inFIGS. 3 and 4). The use of three or more representation levels canprovide substantial resistance to interference by non-GPS signals, as isknown by those skilled in the GPS art. However, a smaller data memory isrequired if only two representation levels (one bit) are used for the Iand the Q signals, and this has a cost advantage in some applications.In one embodiment of the invention, the I and Q RF/IF output signals aredigitized and stored simultaneously, and the data memory length issufficient to hold the entire data sequence needed to achieveacquisition (e.g., 20 ms). With 1023 C/A code chips per millisecond andboth I and Q RF/IF signals being sampled at approximately two samplesper code chip with two bits per sample, approximately 170,000 bits ofstorage are required for the 20 ms signal segment. For convenience, theI and Q data can be considered as stored in separate I and Q memories.When the required data has been sampled, converted, and stored, thereceiver can be turned off and the recorded data can be processed.

In the sequential method of this invention, the stored data is replayed(read) once during the correlation process for each combination of code,code offset, and Doppler shift. In the system of FIG. 4, the sequence ofstored digital data samples are read from memory 33, one at a time. Eachsample of the sequence from memory 33 is multiplied by the correspondingsample of the sequence from the code/Doppler generator 24 in multiplier231, and the result is accumulated in coherent accumulator 232. Thesequence or data segment from the code/Doppler or replica generator iscrafted for a particular code, code offset, and Doppler frequency undertest. The length of the memory sequence so processed is the coherentintegration length, and is typically chosen as one full cycle of the C/Acode, which is 1.0 milliseconds (ms). Several (e.g., twenty) adjacent1.0 ms memory data segments are processed in this manner withoutchanging the replica sequence. After each 1.0 ms segment is processed,the value stored in coherent accumulator 232 represents the crosscorrelation between the 1.0 ms replica and data segments (sequences).This value is squared by squarer 29 and added to non-coherentaccumulator 30. Before the first 1.0 ms segment is processed for a givenreplica signal, non-coherent accumulator 30 is reset to zero so that thefinal accumulated result represents the total correlation score for theparticular code, code offset, and Doppler frequency specified by thereplica signal. Similarly, the coherent accumulator is reset before each1.0 ms segment is processed. Threshold detector 31 monitors thecorrelation score and produces a "signal acquired" signal if the scoreis larger than a specified threshold. Upon receipt of a "signalacquired" signal, control 35 performs a simple peak-search andinterpolation algorithm (described subsequently) to find the bestestimate of the code offset associated with the given code index andDoppler frequency under examination. Control 35 then selects anothercode, code-offset, and Doppler frequency combination and commandsreplica generator to alter the replica signal to reflect this change.The signal acquisition process is repeated for multiple replica signalscorresponding to the codes, code offsets, and Doppler frequencies to besearched, and is stopped when the desired number of GPS signals(different C/A codes) have been acquired. Control 35 then produces as anoutput signal the code indices, estimated offsets, and Dopplerfrequencies associated with the acquired signals.

The I and Q memory data can be processed sequentially, (e.g., processall I data, then process all Q data) using one digital correlator 23 asshown in FIG. 4. Alternatively, the I and Q memory data can be processedsimultaneously using separate digital correlators. In any event, toinsure signal acquisition, I and Q Doppler processing must be performedon both the I memory data, and the Q memory data. Thus there are fourI-Q combinations and they can be processed sequentially with a singledigital correlator, or simultaneously with multiple correlators. In onesequential method, the correlation result for the full memory datasequence with a given replica code index, code offset, and Dopplerfrequency is calculated as follows: First, the coherent and non-coherentaccumulators are reset to zero. Then the I memory data is processed withthe I Doppler replica and the correlation result is accumulated in thecoherent accumulator. The Q memory data then is processed with the QDoppler replica and the correlation result is further accumulated in thecoherent accumulator. The total coherent-accumulation result is thensquared by squarer 29 and added to non-coherent accumulator 30. Thecoherent accumulator is then reset. Next the Q memory data is processedwith the I Doppler replica and the correlation result accumulated in thecoherent accumulator, then the I memory data is processed with the QDoppler replica and the correlation result is inverted (multiplied by-1) and further accumulated in the coherent accumulator. The totalcoherent-accumulation result is then squared by squarer 29 and added tonon-coherent accumulator 30. This process is repeated for each 1.0 msmemory data segment, without resetting the non-coherent accumulatorbetween segments, and requires two full read cycles of both the I and Qmemory data. Acquisition time and energy can be reduced by usingseparate I and Q Doppler generators 242, multipliers 241, and digitalcorrelators 23 to process all four I-Q combinations simultaneously.

In one convenient rendition, code/Doppler generator 24 is comprised ofDoppler I/Q generator 242, satellite code generator 243, offsetgenerator 244, and multiplier 241, as shown in FIG. 4. Doppler I/Qgenerator 242 generates a digital sequence representation of a sine orcosine wave at the desired frequency relative to the sample rate, andcan, for example, be implemented with a Read Only Memory (ROM) driven byan address counter. The frequency and phase (I or Q) of the sinusoid canbe selected by control of the order in which the stored sinusoidalsamples are read (e.g., by address decimation and by selection of thestarting address, respectively). Other digital sinusoid generators arealso known to those skilled in the art. For the GPS C/A codes, a wellknown state machine can be used to generate the C/A code bits in theproper order. The Doppler sinusoid and C/A code values are multiplied bymultiplier 241 to yield the replica signal. Provision for searching allfeasible code offsets is provided by a time offset of the stored codereplica from satellite code generator 243 using offset generator 244.With the state-machine implementation of satellite-code generator 243, aparticular code offset is effected by presetting the state machine tothe associated state before starting the correlation process. Theinitial states, corresponding to different code offsets, can be storedin a ROM and indexed by a simple binary address counter.

In practice, the acquisition search is conveniently performed by firstselecting a code and a Doppler frequency, and then indexing throughdifferent code offsets. Then the Doppler frequency is changed and thedifferent code offsets are re-examined. Non-coherent accumulator 30 canbe implemented as an array of accumulators, one for each candidate codeoffset in a sequence of adjacent offsets. This accumulator organizationallows the scores for adjacent offsets to be simultaneously examined sothat an interpolation algorithm can be applied to find the sub-chipoffset value associated with the actual score peak. If a sequentialinterpolation algorithm is used, the peak offset value can beinterpolated while using only a single accumulator element.

The total acquisition time can be reduced by processing multiple codeoffsets simultaneously. For example, multiple digital correlators 23 canbe used, each driven with a differently delayed version of the replicasignal. The different delays can be implemented with a tapped delay linecoupled to the output of code/Doppler generator 24. The different tapscan then each drive a separate serial digital correlator 23, and theresults from each correlator can be separately squared and accumulatedin associated elements of a non-coherent accumulator array.

An alternative GPS receiver architecture that provides for bothlow-power and fast signal acquisition, according to another aspect ofthe invention, is shown in FIG. 5. The acquisition architecture issimilar to that of FIG. 4, except that the serial digital correlator,serial Doppler and P/N code generators, and serially read memory are nowreplaced with a parallel correlator 36, parallel Doppler and P/N codegenerators 37, and a parallel read memory 33, respectively. One aspectof the invention is a method for massively parallel analog summation inthe parallel correlator. The analog summation, in combination with themassively parallel organization of the memory, replica-generator, andcorrelator elements, provides for the dramatic time and energyreductions in the correlation process. The analog summation result isconverted to digital form by A/D converter 38 which may be combined withsquarer 29, as described subsequently. The parallel architecture alsotakes advantage of conventional low-power complementary metal oxidesemiconductor (CMOS) integrated circuit technology to achieve low-energyuse. Energy use in CMOS circuits is dominated by the charging anddischarging of circuit node capacitances; little energy is used at nodeswhose voltages are static (unchanging) or whose capacitance is small.With this invention the Doppler and P/N code replica generators andregisters, data memory, and parallel correlator are organized tominimize the number of CMOS nodes being charged and discharged duringthe correlation process.

FIG. 6 illustrates the parallel correlation concept and shows waveformsfor the case where the received and replica signals have no Dopplershift. Digitized signal data is written sequentially into the datamemory as it becomes available from the RF/IF section 21 and A/Dconverter 22. Data memory 33 is organized for a massively paralleloutput so that a long sequence of data is available at the outputsimultaneously. Also, a shift register 1004 is loaded with the chosenreplica signal and is organized for a massively parallel output with thesame length as that from the data memory. A sample of thecross-correlation (i.e., the inner product) between the parallel datasequence and parallel replica sequence for a given Doppler frequency,code index and code offset is generated all at once by parallelcorrelator 1000. In parallel correlator 1000, each element of the memorydata sequence is multiplied by the corresponding element of the replicasequence using a corresponding multiplier in an array of multipliers.The multiplier output signals are simultaneously summed to form thecoherent processing result at the correlator output. The coherentprocessing result for an adjacent code offset is generated by shiftingthe replica register one step while holding the memory data stationary.Alternatively, the replica signal can be held stationary while thememory data is shifted.

In one attractive embodiment of the invention, the parallel-output dataand replica sequences are 1.0 ms long and span a single cycle of the C/Acode. The data sequence is generated from the RF/IF output signal bysampling at the rate of approximately two samples per C/A code chip anddigitizing to three levels with an A/D converter using a two-bitsign-magnitude format. This sampling rate and number of levels avoidsaliasing, avoids substantial SNR degradation if the A/D threshold levelsare properly chosen, and yields sequence lengths on the order of 2100samples. The sign-magnitude format allows a simple multiplier to be usedin the multiplier array described above. Other attractive datarepresentations, sampling rates, and parallel-output sequence lengthsare possible, as will become apparent to those skilled in the art.

FIG. 7 shows one method for generating the parallel-output replicasequence. In a code/Doppler generator 1008, a C/A code generator 1001generates the desired C/A code sequence and a digitized sinusoidgenerator 1002 generates the digitized sinusoid sequence with thedesired phase and Doppler frequency. A multiplier 1003 sequentiallymultiplies the code and sinusoid sequences provided by code/Dopplergenerator 1008 to generate the replica sequence, and this sequence isshifted into a parallel-output code/Doppler (replica) register 1004.

The replica sequence is preferably represented in three level (two-bit)sign-magnitude format, or a two-level (one-bit) sign format, as theseformats reduce the complexity and power consumption of the parallelcorrelator with respect to a format that uses more bits. With only threeor fewer levels, the Doppler component in the replica sequence will havehigh harmonic content and these harmonics can falsely correlate with aninput signal. This problem can be avoided by choosing the RF/IF localoscillator frequency so that the RF/IF output signal is substantiallyoffset from zero frequency. By choosing the offset large enough, theharmonics of all resulting necessary Doppler-replica frequencies will bewell above the highest Doppler replica frequency. The harmonic problemis known by those skilled in the design of superheterodyne receivers. Ifdesired, the Doppler-replica harmonic levels can be reduced by usingmore bits in the Doppler and replica sequence representation and morebits per sample in the replica register. However, this will increase thepower consumption and complexity (size) of the signal-acquisitionimplementation.

FIG. 8 shows a short segment of an embodiment of the data memory 33,replica shift register 1004, and parallel correlator 1000 that uses thereplica generation method of FIG. 7. Both the data and replica sequencesuse a two-bit sign-magnitude representation, and since the rows ofelements in each of memory 33, shift register 1004, and correlator 1000are aligned with each other in columnar fashion the sign (S) andmagnitude (M) bits of corresponding samples from the two sequences canbe conveniently fed, in each column, to a corresponding multiplier 1200.With sign-magnitude input values of, for example, either -1, 0, or 1from the two input sequences, each multiplier 1200 generates either -1,0, or 1 as an output signal. A separate digital-to-analog converter 1300converts each digital multiplier output signal to analog form. All D/Aconverter output signals are summed by being furnished to a commonoutput that represents the analog correlation result. The analogsummation is conveniently implemented using charge summation, butalternative analog summation forms are also possible. This summationmethod is spatially efficient, requires very low power, does not requireany analog memory, and is very fast. Another advantage of this parallelcorrelator architecture is that the code and Doppler sequences areprogrammable, allowing a single correlator to rapidly search overmultiple code indices, offsets, and Doppler frequencies in theacquisition process.

FIG. 9 shows one embodiment of one stage of the two-sequence parallelcorrelator of FIG. 8. Multiplier 1200 generates sign and magnitudeoutput bits that control switches 1400 and 1500 in a D/A converter 1300.The switches connect one end of a charge summation capacitor 1100 toeither a positive reference voltage rail or a negative reference rail,or to an output reference voltage rail (e.g., ground). The sum isgenerated by a two step procedure. First the charge-reset line is setlow, closing a charge reset switch 1600 and forcing switch 1400 in eachparallel correlator stage to connect the associated capacitor to theoutput reference rail (ground in FIG. 9). This discharges allcapacitors. Then the charge reset line is set high, opening charge resetswitch 1600 and allowing the data and replica values in each parallelcorrelator stage to control the associated sign and magnitude switches,1500 and 1400, respectively. This two-step process insures that excesscharge does not accrue on the charge summation capacitors over time. Itis advantageous that this parallel correlator can be implemented inlow-cost digital processes by using, for example, metal-metal"cross-over" capacitors and binary electronic switches. Finally, it isadvantageous that the charge summation can be extended across multipleintegrated circuits by simply extending the summation lines.

FIG. 10 shows another method for generating the replica signal. Withthis method, the code and Doppler replica sequences are stored inseparate parallel-output registers. C/A code generator 1001 ofcode/Doppler generator 1010 generates the desired C/A code sequence andthis sequence is shifted into a code replica register 1005. Similarly, adigitized sinusoid generator 1002 of code/Doppler generator 1010generates the digitized sinusoid sequence with the desired phase andDoppler frequency, and this sequence is shifted into the Doppler replicaregister 1006.

FIG. 11 shows an embodiment of the data memory 33, code-replica register1005, Doppler-replica register 1006, and parallel correlator 1000 thatuses the replica generation method of FIG. 10. A separate code register1005 and Doppler register 1006 are used, yielding separate memory-data,code, and Doppler sequences. Sign (S) and magnitude (M) bits ofcorresponding samples from the three sequences are fed to correspondingmultipliers 1201. With sign-magnitude input values of, for example, -1,0, or 1 from the data and Doppler sequences, and -1 or 1 from the codesequence, each multiplier 1201 generates either a -1, 0, or 1 as anoutput signal. The different multiplier output signals are allsimultaneously summed using analog summation as previously described forthe two-sequence parallel correlator.

FIG. 12 shows one embodiment of one stage of the three-sequence parallelcorrelator of FIG. 11. The correlator stage shown is similar to that ofthe two-sequence parallel correlator of FIG. 9 except that themultiplier has an extra exclusive-or gate 1205 to allow the separatecode register bit to impact the sign of the multiplication result.

The two and three sequence parallel correlators of FIGS. 8 and 11 can begeneralized to plural sequence parallel correlators. With respect to thetwo sequence correlator, the three sequence correlator has had one ofthe cross-correlation sequences (the replica) decomposed into twoseparate sequences. This decomposition can be applied to both the dataand replica sequences in general to provide a plural sequence parallelcorrelator or inner-product machine.

"Differential" analog summation in the parallel correlator may havelower noise susceptibility and other advantages. FIG. 13 shows oneconvenient differential summation configuration. In each stage of theparallel correlator, two D/A converters are driven in parallel by thesame sign and magnitude output signals from the associated multiplier.One converter is labeled as positive and its capacitor connects to apositive summation line and the other converter is labeled as negativeand its capacitor connects to the negative summation line. The switchesshown in each converter are implemented as electronic switches, as iswell-known in the art. The two converters operate identically exceptthat the sign switch 1500 in the negative converter is connected in thereverse sense with respect to the sign switch of the positive converter.With the differential method, the difference between positive andnegative summation results must be taken to determine the final coherentcorrelation result. This can be performed by, for example, using ahigh-speed linear or switched capacitor difference amplifier, as isknown by those skilled in the art. Alternatively, the positive andnegative summation results can be separately A/D converted and thentheir difference computed digitally prior to squaring and non-coherentaccumulation.

The three-sequence parallel correlation method of FIG. 11 uses lesspower than the two-sequence method of FIG. 8 because only the 1-bit deepcode register needs to be shifted when developing correlation resultsfor subsequent code offsets; the sequence in the separate Dopplerregister can be held fixed. Shifting a register that is two bits deepconsumes approximately twice the power as shifting a register that isone bit deep. Because register shifting is a dominant power consumerwith the parallel correlation approach, the three-sequence method isadvantageous. On the other hand, the two sequence method uses fewershift register elements (bits) and fewer exclusive-or gates in themultipliers and therefore has a smaller implementation. Theimplementation size difference diminishes as the number of bits ofDoppler replica representation is increased.

D/A converters 1300, shown in FIGS. 9 and 12, utilize series switches tocontrol charging of the charge summation capacitors. An alternativeembodiment for the multiplier and analog converter (D/A) is shown forthe three-sequence case in FIG. 18. An advantage of this embodiment isthat digital logic functions directly drive the charge-summationcapacitors and a series connection of switches is not needed. Thismultiplier-D/A combination is readily implemented in a standard digitalCMOS process. Capacitors 1101 and 1102 are of substantially the samevalue and together provide for a three-level D/A conversion, e.g., -1,0, and +1. The -1 level is invoked by driving both capacitors low(digital ground), the +1 level is invoked by driving both capacitorshigh (digital Vdd), and the 0 level is invoked by driving one capacitorhigh while driving the other low. In a multiplier 2001, exclusive NORgate 1210 and AND gate 1211 multiply the two-bit data and Doppler valuesto form the sign-magnitude product. Exclusive NOR gate 1212 thenconverts the two-bit product to A and B signals that drive the twocapacitors in either the same or different directions. Finally,exclusive NOR gates 1213 multiply the A and B signals by the binary codevalue, while NOR gates 1214 provide for grounding the capacitors so asto discharge them during the reset phase.

For a given length of stored GPS signal, a tradeoff exists between thelength of coherent and non-coherent processing. As the coherentcorrelation length is increased, the SNR of each coherent processingresult is increased but the number of coherent processing resultsavailable for squaring and non-coherent accumulation (integration)decreases. It is known by those skilled in the art that the SNR improvesby 10 dB per decade of coherent processing length increase, butincreases by only about 5 dB per decade of non-coherent processinglength. Thus, for a given memory data length, the pre-detection SNR ismaximized by performing one long coherent correlation. However,unpredicted receiver motion or local oscillator jitter will limit thefeasible length of coherent processing. Also, as the coherentcorrelation length is increased, the bandwidth of the Doppler bins isreduced and this yields more Doppler bins that must be searched. Thearchitectures of FIGS. 4 and 5 readily allow dynamic trading of coherentand non-coherent processing length through control of the replicagenerator, resetting of the coherent and non-coherent accumulators, andbypassing of the squaring functions.

It is usually true that the pre-detection SNR needed for reliabledetection is lower than the SNR needed for accurate correlation peaklocalization (i.e., interpolation). Thus the acquisition time and energycan be minimized by using shorter coherent correlations (and thussearching over fewer Doppler bins that are as wide as the detectionreliability constraints will allow) until a correlation detection isobserved, and then reprocessing the data with longer coherentcorrelation using code offsets and Doppler shifts near the values thatyielded the detection event, to increase the SNR and perform superiorcode offset interpolation. The architectures of FIGS. 4 and 5 readilyallow this two step process.

FIG. 14 shows a section of the noiseless auto-correlation waveform for a1.0 ms C/A-code P/N waveform (not sampled). The auto-correlation valueis very near zero for all offsets of magnitude greater than one chipwidth and is a triangle function (of the offset) for offsets between -1and +1 chip width. In another aspect of the invention, accurate sub-chipcorrelation-peak time estimation is achieved without using samplingrates that are substantially higher than twice the C/A chip rate byusing a sampling rate that is a non-integer multiple of the C/A chiprate. With an integer number N of samples per C/A code chip, thediscrete-time auto-correlation sample value remains substantiallyconstant over an input-signal time offset variation of 1/N of a C/A codechip period when the receiver bandwidth is substantially wider than theGPS signal bandwidth. This is a form of quantization and yieldssubstantial quantization error if N is not large. For example, an offsetmeasurement accuracy of one-tenth of a C/A code chip would require asampling rate of N=10 times the C/A code chip rate. With a more-limitedreceiver bandwidth, the auto-correlation sample value varies withinput-signal time offset, but not necessarily linearly with inputoffset. If a non-integer multiple sampling rate is used, then the sampleinstants precess or roll in chip-relative position as a sequence ofsignal C/A code chips is sampled. This precession allows the calculatedauto-correlation value to change substantially linearly withinput-signal time shift, and with much smaller time shifts. Although theabove discussion has focused on the auto-correlation of the binary C/Acode, the sample precession method is applicable to multiple-levelsignals in general.

With a replica code index and Doppler frequency that matches that of anoisy input signal, the serial or parallel correlator will calculatenoisy samples of the shown auto-correlation function. By choosing thesignal sampling rate so that there is an integer number of samples(e.g., 2183) per 1.0 ms C/A code period, correlation results from, forexample, twenty consecutive 1.0 ms data memory sequences (using the samereplica code offset) will yield twenty noisy samples of the same pointon the C/A-code auto-correlation waveform. During signal acquisition,the replica Doppler frequency and phase rarely match that of the signalexactly so that squaring of the coherent correlation result is necessaryto guarantee a positive correlation result. FIG. 14 also shows typicalsquared 1.0 ms coherent correlation sample points (x) calculated forseveral code offsets near that of the received signal, and with codeindex and Doppler frequency and phase that match that of the inputsignal. By averaging the squared samples from the same code offset, alower-noise estimate of the corresponding auto-correlation value isgenerated. Typical averaged auto-correlation samples are also shown inthe figure. This averaging is the so-called non-coherent accumulation.The phase of the sample grid, relative to the triangle function, dependson the phase of the received waveform relative to the input A/D samplingclock.

In the waveform of FIG. 14, the time of the auto-correlation peak is aparameter of interest, and this peak time is usually not on the samplinggrid. However, using the averaged auto-correlation values surroundingthe peak time, the peak time can be estimated by interpolation. Variousinterpolation methods are known to those skilled in the art, but onemethod involves ordering the averaged auto-correlation samples accordingto increasing code offset and then searching for the two largestadjacent entries. The left entry (the earlier one) and its left neighbor(points B and A respectively in the figure) together define a line,while the right entry and its right neighbor (points C and Drespectively in the figure) together define another line. These twolines intersect somewhere between the two original end points (A and Din the figure) when the SNR is high enough, and the time associated withthis intersection is the estimated correlation peak time. This methodrequires solution of two simultaneous equations, may be used with eitherthe serial or parallel correlator methods, and would be computed bycontrol 35 shown in FIG. 4 or FIG. 5.

FIG. 15 shows a low-power parallel-correlator based acquisition systemwith full I/Q processing in which the RF/IF section (not shown)generates two quadrature output signals which are passed to input A/Dconverters 22 and 56. Control 35 applies power to the RF/IF section andto input A/D converters 700 and 701. A/D converter 56 generates sampledI data which is stored in the data I memory, while A/D converter 22generates sampled Q data which is stored in the data Q memory.

In one embodiment of the system, the input A/D converters sample the Iand Q input signals periodically at approximately 2.183 million samplesper second, yielding 2183 samples per C/A code cycle and approximately2.1 samples per C/A code chip. The samples are quantized to 3 levels anduse two-bit sign-magnitude representation, as described earlier. The Iand Q data-storage memories are each large enough to store the entiredata segment needed for signal acquisition. For nonmilitary utilization,this is usually 20 ms. After 20 ms (2183×20 samples) of I and Q datahave been stored, power is removed from the RF/IF section and from theinput A/D converters, and acquisition processing is begun. The I and Qdata memories are each organized as twenty rows of 2183 two-bit samples,with 2183 simultaneous outputs (one full row). Four separatethree-sequence parallel correlators are used, labeled II, IQ, QI, andQQ. Together these comprise a complex parallel correlator 70. Theparallel correlators each use sequence lengths of 2183 samples. The Idata memory drives the II and IQ correlators, while the Q data memorydrives the QI and QQ correlators. Also, the I Doppler register drivesthe II and QI correlators while the Q Doppler register drives the IQ andQQ correlators. The order of data processing is set so that all desiredreplica code offsets are examined for a given replica code index andDoppler frequency before the code index or Doppler frequency is changed.Furthermore, all desired code offsets are processed for a given row ofstored I and Q data before the row is changed. This will most likelyminimize the combined memory reading and code-register shifting energyuse. Other memory-row, code-index, and Doppler frequency processingorders are also possible, and the order can be chosen to minimize energyconsumption. The II and QQ coherent processing results are summed toform II+QQ by connecting the associated summation lines, and the QI andIQ coherent processing results are summed by connecting their associatedsummation lines. For proper I-Q processing, either the QI or the IQresult must be inverted, and this can be accomplished by, for example,reversing the sense of all sign switches in all analog converters of thechosen parallel correlator. In this embodiment, IQ-QI is formed.

With GPS signals, the SNR of each II+QQ and IQ-QI signal is typically 0to 4 dB. These signals are adequately represented with only a few levels(e.g., -1, 0, 1) and are converted to digital form by A/D converters 700and 701, and then separately squared by squarers 702 and 703. Thesquared signals are summed by digital adder 46 and stored in accumulator44. Accumulator 44 has a separate storage position associated with eachcandidate code offset, and correlation results for each code offset withdifferent received data segments are accumulated in the associatedstorage position.

FIG. 16 shows one embodiment of a squaring flash A/D converter thatcombines the A/D and squaring functions. Here two comparators 61 and 62determine if the analog value is above, below, or between the twothreshold values associated with analog representation levels -1, 0, +1.A logic unit 63 then maps the comparators' joint output state to theappropriate squared digital value. If desired, this squaring flashconverter is readily generalized to more than three representationlevels by adding more comparators and threshold voltages.

The sampling rates, stored sequence lengths, and simultaneous memoryoutputs lengths can be chosen to best fit the GPS application. Forexample, by storing the entire data segment needed for signalacquisition, the RF/IF section can be turned off after the shortestpossible time (commensurate with receiving the needed data segment). Thestored data set can then be used for the processing of all SCD bins inthe acquisition search. Alternatively, a shorter memory can be used atthe expense of having to keep the RF/IF section on for a longer time.For example, another attractive embodiment uses a "Ping-Pong" memoryorganization and is illustrated in FIG. 19. Here, the I and Qdata-storage memories 192 and 194, respectively, are each only 2.0 mslong and are organized as two parallel-output 1.0 ms rows (e.g., 2183samples in each row, as before). In both the I and Q data channels, onedata row is accessed in parallel, for parallel correlation, while theother data row is being written with digitized data arriving from theRF/IF section. The entire I and Q data sequence lengths are eachprocessed in 1.0 ms segments. The entire 20 ms data set is thenprocessed for a single SCD bin in approximately 20 ms. Because theentire data sequence was not stored, the RF/IF section must be energizedand produce another entire sequence if another SCD bin is to be tested.On the average, this increases the time that the RF/IF section must bekept energized during signal acquisition. However, the reduction inmemory size can be dramatic. In some applications (e.g., militaryreceivers under severe jamming conditions), the GPS SNR is very low andthe data sequence length needed for acquisition may be so long thatstorage of the entire sequence is impractical. Under such circumstances,the Ping-Pong acquisition architecture is advantageous.

In some applications it will be advantageous to further reduce the sizeof the data-storage memories. With the two-segment (Ping-Pong) memoryorganization, a long continuous stream of receiver-output data can beprocessed. At the expense of a longer time to acquisition, thetwo-segment I and Q data memories 192 and 194 (FIG. 19) can be reducedto single-segment memories by eliminating one of the 1.0 ms sectionsfrom each memory. With this reduction, time-adjacent 1.0 msreceiver-output segments are not processed. Instead, only every other1.0 ms segment is captured in memory and processed, and this lengthensthe time elapsed for processing a given number of received signalsegments.

To further reduce the memory size and the complexity of the parallelcorrelator, the length of the parallel correlator and the associatedregisters and memory segments can all be made smaller than the desiredcoherent processing length. The desired coherent processing length canbe achieved by processing multiple shorter data segments and coherentlycombining their results. For example, a 1.0 ms coherent processinglength can be achieved by using a single 0.5 ms long parallel correlatoron two adjacent 0.5 ms data segments and adding their results digitally,after A/D conversion. Various combinations of correlator length,coherent processing length, numbers of memory segments, and code-offsetsearch width are possible.

When the cost, size, and power dissipation of a standard serial-inputserial-output (SISO) memory are sufficiently low, it may be advantageousto store the entire necessary I and Q data sequences in such memory,turn off the RF/IF receiver section, and use this memory in combinationwith the Ping-Pong or single-segment memory architectures to examine alldesired SCD bins. The entire I and Q data sequences are read from theSISO memory once for each SCD bin tested, and this reading process willconsume energy. However, because the RF/IF section is turned off, thesystem energy use will be reduced if the power dissipation of a SISOmemory is sufficiently low relative to that of the RF/IF receiversection. Other stored sequence lengths, simultaneous memory outputlengths, and sampling rates are also attractive.

In the two-sequence or three-sequence parallel correlator embodiments ofFIGS. 8 and 11, the product of associated data and replica samples isperformed in parallel within the correlator. This allows differentDoppler bins to be tested for acquisition without having to collect newreceived data. FIG. 20 shows an alternative embodiment of one stage of aparallel correlator and the associated parallel memory where the dataand Doppler samples are multiplied together by multiplier 2001 prior tostorage in the parallel memory 2002. This is advantageous because themultipliers in the parallel correlator are thus simplified. Eachcorrelator multiplier now multiplies its associated stored sample byjust a single associated code bit. The combined multiplier and D/Aconverter for this correlator embodiment is similar to that shown inFIG. 18, except that the data-Doppler multiplier and A-B signalconverter, comprised of exclusive NOR gates 1210 and 1212 and AND gate1211, is moved out of the correlator and ahead of the data memory. Thedisconnected inputs to exclusive NOR gates 1213 are coupled to the datamemory outputs. With this embodiment, new data must be stored if adifferent Doppler bin is to be tested for acquisition, but this is not adisadvantage in some applications.

The combined parallel correlator and parallel memory shown in FIG. 20can be advantageously used to form a simplified full I-Q processing GPSacquisition engine. In the full I-Q acquisition engine shown in FIG. 19,there are four separate parallel correlators coupled to data I and Q andDoppler I and Q registers (parallel memory), as well as to the parallelcode register. An alternative full I-Q embodiment that uses the simplercorrelator and memory organization of FIG. 20 is shown in FIG. 21. Inthis embodiment, the four parallel data-Doppler memories hold the II,QQ, IQ and QI data-Doppler product sequences, respectively. Eachparallel memory can be organized as a Ping-Pong or single-segmentmemory, as described earlier. There is no separate data and Dopplerstorage. Each parallel memory connects to an associated parallelcorrelator, and each parallel correlator also connects to the singleparallel code register. The reduced interconnection between memorysections and correlator sections allows this full I-Q processorembodiment to have a simpler (i.e., smaller and less costly)integrated-circuit implementation.

The complexity of the embodiment of FIG. 21 can be still further reducedby performing the II+QQ addition and the QI-IQ subtraction before datastorage. Prior to storage, the addition and subtraction output signalscan often be rounded or truncated to two bits with little SNRdegradation. Then, as shown in FIG. 22, only two data-Doppler memories(one for II+QQ data, the other for QI-IQ data), two parallelcorrelators, and one code register are needed. Each data-Doppler memorycan be organized as a Ping-Pong or single-segment memory, as describedearlier. Other organizations of the parallel correlator, parallelmemory, and the sequence multiplications are also possible andadvantageous.

FIG. 17 shows a combined data-memory, replica-generation,parallel-correlator organization (for just the II correlation process)that reduces the number of code register shifts to 2183 shifts percode/Doppler combination and eliminates need for any non-coherentaccumulation RAM. The trade-off is that the input memory must now besegmented to allow all stored data to be simultaneously accessed. Thesedata are applied, in 1.0 ms blocks, to separate parallel correlators.These parallel correlators are all driven by the same code and Dopplerreplicas, and they simultaneously (in one full code cycle) generate thetwenty separate 1.0 ms correlation sequences that were previouslyaccumulated time sequentially. Separate squaring A/D converters convertthe analog correlation sequences to digital form where an adder tree 80forms the accumulated correlation sequence. The accumulation RAM can becompletely avoided by performing the peak search and interpolationprocess on the accumulated correlation sequence as it is beinggenerated. In this arrangement, the code and Doppler registers now drivetwenty loads where they previously drove one, which may negate some ofthe energy reduction.

When the parallel correlator is used with data and replica registers forthe two-sequence case, or with data, Doppler, and code registers for thethree-sequence case, it is advantageous to minimize the energy used inloading (writing) the various registers. In another aspect of thepresent invention, loading of the various registers is made a low energyoperation by using a scanner shift register to determine which data,Doppler, and code register stages are to be written at each instant.FIG. 23 shows a scanner shift register 304 in combination with the dataregister 300, Doppler register 301, and code register 302 for thethree-sequence case. In one method of operation, samples arrivesequentially for each of the digital data, Doppler, and code sequencesand the sample rates are the same. As the samples for each respectivesequence arrive, they are written into corresponding sequentiallocations in their respective registers. In synchronism with thearriving samples, a single logical 1 is shifted along the binary scannerregister, enabling the writing of the arriving samples intocorresponding sequential locations of the respective registers. Thewriting operation is very low energy because only two adjacent scannerregister locations change their stored value at each shift, and none ofthe respective registers is shifting during the writing operation.

While the disclosed invention is applicable to GPS C/A signalacquisition, it can also be used to dramatically reduce both the timeand energy required to directly acquire the P(Y) military GPS signalswithout first acquiring the C/A signals. By altering the satellite-codegenerator component of the code/Doppler generator in FIGS. 4 or 5 togenerate the P(Y) code instead of the C/A code, the methods andarchitectures disclosed herein become applicable to the P(Y) case.

While only certain preferred features of the invention have beenillustrated and described, many modifications and changes will occur tothose skilled in the art. It is, therefore, to be understood that theappended claims are intended to cover all such modifications and changesas fall within the true spirit of the invention.

What is claimed is:
 1. In a direct sequence spread spectrum receivercapable of receiving a plurality of signals from respectivetransmitters, apparatus comprising:a tuner for receiving at least one ofsaid signals and producing an intermediate frequency signal, said tunerincluding a converter for sampling said intermediate frequency signal; afirst and a second memory for alternately storing sampled segments ofsaid intermediate frequency signal; a processor for alternately readingand processing said segments; a controller for controlling the alternatestorage and the alternate processing of said segments.
 2. The receiverof claim 1, wherein:said receiver comprises a GPS receiver and saidtransmitters comprise GPS transmitters carried by satellites.
 3. Thereceiver of claim 1, wherein:said processor comprises a correlator. 4.In a direct sequence spread spectrum receiver capable of receiving aplurality of signals from respective transmitters, a method comprisingthe steps of:receiving a first time segment of at least one of saidsignals; converting said time segment of said signal to amultiple-sample digital form; storing said multiple-sample digital formin a memory, said memory having an output signal comprising saidmultiple-sample digital form; applying said multiple-sample digital formsimultaneously from said memory into a processor; and processing saidmultiple-sample digital form; wherein time segments of said signalsarriving at said receiver subsequent to said first time segment areskipped during said processing step.
 5. The method of claim 4,wherein:said receiver comprises a GPS receiver and said transmitterscomprise GPS transmitters carried by satellites.
 6. The method of claim4, wherein:said processing comprises the step of correlating the firsttime segment with local pseudo random noise code sequences.
 7. Thereceiver of claim 1, wherein:said processor is adapted to produce acoherent result capable of being combined with other coherent resultsfrom said processor, wherein the combined coherent results exceed athreshold for indicating identification of said signal.
 8. The receiverof claim 1, wherein:said segments are representative of one millisecondof said intermediate frequency signal.
 9. The receiver of claim 1,wherein:said processor is adapted to read a stored segment of saidintermediate frequency signal from said first memory while said secondmemory stores another segment of said intermediate frequency signal fromsaid tuner.
 10. The receiver of claim 1, wherein:said processor isadapted to process sequential segments of said intermediate frequencysignal until a data set is processed.
 11. The receiver of claim 1,wherein:each of said first and second memories is adapted to providedigital data samples of intermediate frequency signal segments storedtherein, in parallel.
 12. The receiver of claim 11, wherein:lengths ofsaid first memory, said second memory, and said processor are equal. 13.The receiver of claim 1, further comprising:a third memory, a fourthmemory, and a second processor, wherein:said third memory, said fourthmemory and said second processor substantially duplicate said firstmemory, said second memory and said processor, such that said firstmemory, said second memory and said processor are adapted to receivein-phase intermediate frequency signal segments, and said third memory,said fourth memory and said second processor are adapted to receivequadrature phase intermediate frequency signal segments.
 14. Thereceiver of claim 1, further comprising:a front-end memory for storingsaid intermediate frequency signal segments from said tuner, said signalsegments comprising a data set, and for supplying said first memory andsaid second memory with time segments from the stored signal segments,wherein power to said tuner is inhibited after said data set is sampledand stored in said front-end memory.
 15. The receiver of claim 1,further comprising:a replica signal generator for providing to saidprocessor a replica signal having a replica code and a code offset. 16.The receiver of claim 15, wherein:said processor is adapted to comparesaid replica signal to said intermediate frequency signal segments toproduce a coherent result.
 17. In a direct sequence spread spectrumreceiver capable of receiving a plurality of signals from respectivetransmitters, apparatus comprising:a tuner for receiving at least one ofsaid signals and producing an intermediate frequency signal, said tunerincluding a converter for sampling said intermediate frequency signal toprovide segments thereof; a memory for storing every other segment ofsaid intermediate frequency signal; a processor for reading andprocessing the intermediate frequency signal segments; and a controllerfor controlling the storage and the processing of said intermediatefrequency signal segments.
 18. The receiver of claim 17, whereinsaidreceiver comprises a GPS receiver, and said transmitters comprise GPStransmitters carried by satellites.
 19. The receiver of claim 17,wherein:said processor comprises a parallel correlator.
 20. The receiverof claim 17, wherein:said processor is adapted to produce a coherentresult capable of being combined with other coherent results from saidprocessor, wherein the combined coherent results exceed a threshold forindicating identification of said one of said signals.
 21. The receiverof claim 17, wherein:each of said segments is representative of arespective one millisecond portion of said intermediate frequencysignal.
 22. The receiver of claim 17, wherein:said processor is adaptedto process sequential segments until a data sequence is processed. 23.The receiver of claim 17, wherein:said processor is adapted to read inparallel said segments in said memory.
 24. The receiver of claim 17,further comprising:a front-end memory for receiving a data segment fromsaid tuner, and for supplying said memory with a portion of said datasegment, wherein power to said tuner is inhibited after said datasegment is stored in said front-end memory.
 25. The receiver of claim17, further comprising:a replica signal generator for providing to saidprocessor a replica signal having a replica code and an offset code. 26.The receiver of claim 25, wherein:said processor is adapted to comparesaid replica signal to said intermediate signal segments to produce acoherent result.
 27. In a direct sequence spread spectrum receivercapable of receiving a plurality of signals from respectivetransmitters, a method comprising the steps of:(a) receiving a segmentof a signal; (b) converting the received segment of said signal to onemultiple-sample digital segment; (c) storing said one multiple-sampledigital segment in a memory; (d) transferring said one multiple-sampledigital segment from said memory to a processor; (e) storing a secondmultiple-sample digital segment in said memory, wherein said one andsaid second multiple-sample digital segments are alternately stored insaid memory and transferred from said memory for processing; (f)processing alternately the transferred one multiple-sample digitalsegment and the transferred second multiple-sample digital segment; and(g) repeating steps (a) to (f) until a data segment sufficient forsignal acquisition is processed.
 28. In a direct sequence spreadspectrum receiver capable of receiving a plurality of signals fromrespective transmitters, a method comprising the steps of:(a) receivingat least one of said signals; (b) converting said signal to amultiple-sample digital signal, said multiple-sample digital signalcomprising a data segment; (c) storing said multiple-sample digitalsignal in a front-end memory; (d) inhibiting power to said tuner; (e)transferring a portion of said multiple-sample digital signal to asegment memory having a parallel sample output; (f) supplying saidportion from said parallel sample output to a parallel processor; (g)processing said portion by calculating the inner product of said portionand a C/A replica code sequence; and (h) repeating steps (e) and (g)until said data segment is processed.